A device that measures a phase or a phase difference of periodical input signals by using a digital circuit (hereinafter, referred to as a phase measuring device) is one of the most fundamental components in wireless communications, high-frequency signal processing, and high-accuracy frequency measurement. In particular, the phase information obtained by a phase measuring device can be differentiated by time to calculate a frequency, so the device is used as a frequency counter as well.
In wireless and wired communications, with an increase in speed and capacity of communications in recent years, phase noise in a reference signal source has become a factor that restricts the performance. Thus, in order to evaluate the reference signal source, a device that measures a time history of a phase of a periodical input signal (hereinafter, referred to as a phase noise measuring device) or the like has been used.
Further, a phase measuring device that measures a phase difference between two periodical input signals (hereinafter, such a device may also be referred to as a “phase difference measuring device”) has wide applications as an element constituting a large number of measuring and/or controlling apparatuses.
For example, in a laser heterodyne displacement measuring device, a phase difference measuring device has been used for demodulating a displacement of a measurement object from an optically modulated signal.
In a power control system as well, periodically changing alternating current power, voltage, and current signals need to be measured, so a phase difference measuring device has been incorporated as part of the system. In particular, with the requirement for size reduction of the power control system, the phase difference measuring device is desired to have a simple configuration while maintaining a certain level of accuracy.
A phase-locked loop (hereinafter, referred to as a PLL circuit) is a circuit widely used in communication equipment and measuring devices. The PLL circuit has, as its internal element, a phase difference measuring device that measures a phase difference.
Some of physical quantity measurement sensors, such as an angular velocity detection sensor (also called a gyro sensor), have a PLL circuit incorporated therein, so it can be said that these sensors similarly include a phase difference measuring device portion.
In addition, a phase difference measuring device that measures a phase difference of digital pulse signals (signals with square waveforms) has widely been used primarily in communication equipment.
Such phase measuring devices and phase difference measuring devices have their measurement accuracy, resolution, dynamic range, and other performance affecting the ultimate system performance. Research and development have thus been conducted to improve both the performance and handiness of these measuring devices.
Recently, particularly from the convenience for interface with computers and for implementation, devices that use an A/D converter to convert an input signal into digital data and then measure a phase or phase difference through digital processing (hereinafter, referred to as digital phase measuring devices or digital phase difference measuring devices) have become available.
As a way of measuring a phase or phase difference of periodical input signals through digital data processing, several techniques have conventionally been known, which can be categorized broadly as a demodulation method, a counting method, or a zero-crossing method.
Firstly, the demodulation method will be described.
In the demodulation method, a reference signal is generated inside a circuit, and is multiplied with an input signal to detect a phase of the input signal. This processing can be performed on a periodical input signal to implement a phase measuring device.
With this method, although phase measurement is generally possible with high accuracy, demodulation becomes impossible when the input signal greatly differs in frequency from the reference signal. Further, the measurement accuracy would worsen when the input signal varies in amplitude or suffers distortion.
Besides this method of performing multiplication with a reference signal, there are a method of performing discrete Fourier transform, and a method of generating a quadrature signal by Hilbert transform and calculating a phase by arctangent calculation. These methods can also be categorized as the demodulation method and involve similar problems.
Next, the counting method is a technique known since a long time ago, in which the number of times a periodical signal crosses zero is counted using a counter and a phase is calculated from the counted value. Although it can be implemented with a very simple circuit configuration because of its principle, it can measure only a phase that is an integral multiple of the signal frequency, which poses a limitation in accuracy. Various modifications have thus been made to improve the accuracy of the counting method.
For example, a technique of disposing a PLL circuit in a preceding stage and multiplying the frequency of the periodical input signal before applying the counting method has been proposed. This technique makes it possible to amplify a small phase change in the input signal, to thereby improve the accuracy. The PLL circuit, however, has a limited response speed, so the measurement reliability decreases when the frequency of the periodical input signal varies severely (when the phase swings wildly).
Further, a method of combining a simple counting method with a counting method using a clock of higher frequency to improve the accuracy has been proposed as well. This however requires a high frequency clock, and also leads to a complicated circuit configuration and complex signal processing.
Furthermore, in order to correct a counted value that is always an integer value, there is a technique of calculating and correcting a fractional portion by performing linear interpolation before and after the zero-crossing point. Even if such correction is performed, the technique uses only the counted value at the end of a certain measurement time (also called a gate time), so there is a limit on the measurement resolution.
In the zero-crossing method, the time of the point (zero-crossing) when a periodical input signal has crossed zero is measured, and a phase of the signal is computed based on that time.
Specifically, making use of the fact that the time interval at which a signal crosses zero is proportional to the reciprocal of the frequency, the phase difference of the periodical input signal is calculated on the basis of the data accumulated in a memory.
With this method, however, among the data accumulated in the memory, data between adjacent two measurement points is used to estimate the phase, so the measurement time would be limited by the memory capacity.
Further, the time when the signal has crossed zero needs to be converted into a phase, so the signal processing and others increase the calculation load, making it difficult to implement real-time processing.
In order to enhance the performance of the phase measuring device, it is effective to combine a plurality of techniques so as to compensate for their shortcomings. In particular, combination of the counting method capable of addressing high-speed phase changes with the zero-crossing method capable of supporting highly accurate phase measurement is very effective, because it can configure a measuring device that satisfies both high-speed processing and a high degree of accuracy without the use of complicated digital processing as in the demodulation method.
From this standpoint, for example, Patent Document 1 discloses a phase difference measuring device applied to a laser heterodyne interferometer, wherein the counting method employing an up-down counter and the zero-crossing method using a triangular wave generated from an input signal are combined to detect a phase difference.
Further, Patent Document 2 discloses a phase-locked loop having a digital phase difference measuring unit used as part of the circuit, wherein an input signal is A/D-converted and then subjected to processing in a clock generating unit, a phase comparing unit, and a phase correcting unit, to thereby obtain a phase difference between the input sine wave signal and an internally held clock.
The principle of this phase difference measuring unit is that an input signal is initially digitized by the A/D converter to generate an input signal digital value. Next, the clock generating unit generates a “code clock” that expresses the positive or negative of the digital value of the input signal.
Next, in the phase comparing unit, the code clock is used to perform counting based on a high-speed “count clock” held inside. At the same time, in the phase correcting unit, linear interpolation is performed for the data before and after the zero-crossing point of the input signal digital value to calculate a phase correction value, and the output value from the phase comparing unit and the phase correction value are added up to thereby obtain a desired phase difference.
Patent Document 3 discloses, as a phase error (synonymous with the phase difference as used herein) detecting device, a method of A/D-converting an input signal and detecting and correcting a phase error. Detection of a phase error is implemented by: an equalization unit having a predetermined equalization characteristic; a binarization unit that binarizes the signal output from the equalization unit; and an arithmetic unit that calculates a desired phase error signal, by metric computation, from the outputs from the equalization unit and the binarization unit. For implementing the correction, it is determined whether the past phase error history falls within a predetermined range and, when an error out of the range has been detected, correction is performed to cause the error to fall within that range.
Patent Document 4 discloses a frequency measuring method and apparatus, in which amplitude values before and after the zero-crossing point of a periodical input signal are subjected to interpolation, to sequentially calculate the times of the zero-crossing points, and the frequency of the periodical input signal is calculated from the reciprocal of the difference of the times of the zero-crossing points. This method is also included in the “zero-crossing method”.